Add new Linker APIs

[Robbepop]

Adds the following Linker APIs:

• Linker::allow_shadowing: configures the Linker to allow to shadow previous definitions without errors.
• Linker::instance: convenience method to define all exports of an Instance.
• Linker::alias_module: convenience method to add a module aliasing name.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 81.34%. Comparing base (d3e6147) to head (d197a97).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/wasmi/src/linker.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1281      +/-   ##
==========================================
- Coverage   81.34%   81.34%   -0.01%     
==========================================
  Files         306      306              
  Lines       25248    25252       +4     
==========================================
+ Hits        20539    20540       +1     
- Misses       4709     4712       +3     

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